1. Field of the Invention
This invention relates to an ink to be used in an ink jet recording method in which ink is ejected from a recording head through action of thermal energy, an ink cartridge, an ink jet recording method and an ink jet recording apparatus using such an ink. More particularly, the present invention relates to an ink containing a coloring material having a specific structure that can improve the ink ejection stability and shows a high storage stability as well as an ink cartridge, an ink jet recording method and an ink jet recording apparatus for using such an ink.
2. Related Background Art
Ink jet recording is a recording method in which ink is applied to a recording medium such as a sheet of paper to form an image thereon. It enables printing with a less expensive apparatus, a high resolution and a high speed. Ink jet recording has become very popular in recent years as means for forming color images with ease and there is an increasing demand for higher quality color images.
Currently, various pigments are being used as coloring materials to be used for ink jet recording. Since azo pigments particularly show an excellent coloring ability and are advantageous in terms of cost, various developments are under way.
For example, C. I. Pigment Yellow 74, which is a monoazo pigment, shows a high coloring ability and can produce sharp and well-developed color images so that it is being popularly used as coloring material for yellow ink used in an ink jet recording method.
However, C. I. Pigment Yellow 74 is particularly thermally unstable among numerous pigments. For this reason, pigment particles of C. I. Pigment Yellow 74 are grown or crystals thereof are transformed to change the color tone, lowers color density and/or produce dull colors or opaque colors depending on heat conditions in heating and kneading in a kneading step of resin and pigment. Techniques for improving the thermal stability of pigments include, for example, mixing an azo pigment that is poorly thermally stable with a thermally stable insoluble monoazo pigment, an insoluble disazo pigment or a condensed (poly-) azo pigment with a specific ratio.
It is also known that a popular method called co-coupling is effective for manufacturing fine pigments (see, e.g., Japanese Patent Publication No. 60-36224 and Japanese Patent No. 3055673). There have been made proposals relating to monoazo pigment compositions containing two or more monoazo pigments of different types that are obtained by subjecting bases and couplers of different types to a coupling reaction when synthesizing a yellow monoazo pigment (see, e.g., Japanese Patent Application Laid-Open No. 2000-63694 and Japanese Patent Application Laid-Open No. 2001-324837).
Proposals for manufacturing C. I. Pigment Yellow 74 with an improved thermal stability by dispersing a thermally stable pigment in a reaction vessel, synthesizing C. I. Pigment Yellow 74 by means of a diazotizing/coupling reaction and subsequently maturing the reaction mixture (pigment formation) are also known (see, e.g., Japanese Patent Application Laid-Open No. 2004-182952).
Various other proposals have also been made for the purpose of improving the reliability (storage stability, ejection stability and so on) and the high image quality (fixing ability, marker resistance and so on) of an ink jet recording ink prepared by using pigments as coloring materials. For example, techniques of using an ink containing a compound having an alkylene oxide chain to improve the ejection performance and prevent the ink ejection micro-orifices arranged at the recording head from clogging have been proposed (see, e.g., Japanese Patent Application Laid-Open No. H9-165538). Inks prepared so as to contain a 1,2-alkanediol or a glycol monoether for the purpose of improving the permeability of ink have also been proposed (see, e.g., Japanese Patent Application Laid-Open No. 2001-354888).
In particular, there are recently made proposals for solving the above identified problems by designing properties and contents of water-soluble organic solvents and the additives contained in ink along with coloring materials.
The inventors of the present invention conducted evaluation of ejection stability of ink by loading an ink jet recording apparatus adapted to eject ink from the recording head thereof through action of thermal energy (to be also referred to as thermal type ink jet recording apparatus hereinafter) with an ink containing a monoazo pigment as coloring material. As a result, it was found that various phenomena occur as ink is ejected continuously, which phenomena include that the volume of the ejected ink gradually decreases and that impact position of ink on the recording medium becomes shifted. In short, the ejection stability is gradually degraded.
In view of the findings, the inventors of the present invention looked into the causes of degradation of the ejection stability and observed how the pigment dispersion that is contained in an ink changes as the ink passes through an ink flow path of the recording head and is ejected from the ejection orifices.
As a result, it was found that the dispersion breakdown of the pigment dispersion that is caused by the heat applied to it when ink droplets are ejected occurs rapidly and excessively particularly in the case of monoazo pigment dispersions that show a low thermal stability among numerous pigment dispersions.
In other words, the ejection stability is degraded not because of the clogged ink ejection orifices, which have hitherto been known. When heat is applied to an ink containing thermally unstable monoazo pigments as coloring material to raise the ink temperature, dispersion breakdown of the pigment dispersion takes place rapidly and excessively. Then, as a result, deposits adhere to the inside of the ink flow path and vicinity of the ejection orifices to consequently block the ink flow path of the recording head and degrade the ejection stability.
The above-described phenomenon has not been known so far and the present invention is based on this finding.
Thus, it is an object of the present invention to provide an ink to be used in an ink jet recording method in which ink is ejected from a recording head through action of thermal energy that can improve the ink ejection stability.
Another object of the present invention is to provide an ink set, an ink cartridge, an ink jet recording method and an ink jet recording apparatus using such an ink.